In the prior art, it is known that profound changes in the optical properties of liquid crystals, particularly for optical display purposes, can be obtained not only by applying thereto electric fields (electro-optical effects), but also by subjecting them to temperature variations (thermo-optical effects).
A first method consists in forming a cell by disposing between two transparent plates a uniformly oriented, and therefore completely transparent, thin layer of a material in the smectic phase. A light beam, generally chosen from the near infra-red range and intensity-modulated by the video signal transmitting the image to be written, scans the cell point-by-point. When the energy supplied at a point by the beam has been sufficient to locally change the thin layer from the smectic to the isotropic liquid phase, a disordered texture appears during the ensuing cooling back of the layer, producing optical scattering properties of this point; on the other hand, the points where the luminous energy of the modulated beam has been insufficient to bring about the fusion retain the uniform ordered structure and remain transparent. The image is thus written in the form of scattering spots on a transparent background.
It is possible to erase selectively a part of the recorded image. For this purpose, a unidirectional or preferably alternating electric field is applied to the whole layer, while the light beam, which again scans the cell, again brings into the isotropic liquid phase those spots which are to be erased. On the subsequent return to the smectic phase, the field orients the molecules, which are restored to the initial transparent ordered structure.
This method involves the use of a high-power radiation source, because the quantity of heat which must supplied to the layer to bring it into the scattering condition is considerable, since, generally, the transition from the smectic to the liquid phase is not direct, but takes place by way of a nematic phase. Consequently, the temperature difference which must be passed through is considerable, and, in addition, it is necessary to supply the latent heats for the change of state from the smectic to the nematic phase and from the nematic to the isotropic liquid phase.
It has therefore been proposed, in order to reduce the power required from the light source, to replace the material in the smectic phase by a mixture of materials in the nematic and cholesteric phases. The transition into the isotropic liquid phase is then direct. However, selective erasure of the image is no longer possible, because when these mixtures are subjected to an electric field, they become reoriented independently of any heating; consequently, when the electric field is applied, it reorients the whole of the layer, whereby the erasure is made total.
Finally, in a third method by means of which it is also possible to reduce the power required from the radiation source, use is made of the property possessed by certain materials having a smectic phase of exhibiting, when cooled from the isotropic liquid phase, a texture whose disorder is controllable by the electric field applied to the layer. The cell is then swept by a beam of constant intensity, the video signal modulating the applied electric field. It is possible by this method to omit the beam modulator, which results notably in a substantial reduction in the power required from the light source.
It nevertheless remains true that, by reason of the fact that these various methods involve at least one change from one phase to another, with the corresponding latent heat required for the change of state, they necessitate the application of considerable energy to the thin layer by the light beam.